Covalent Bonding and Properties
Covalent bonding involves atoms sharing electrons, typically between non-metals. For the ESAT, you must be able to predict a substance's physical properties by identifying whether it consists of small, discrete molecules or is a single giant network of atoms.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
- Simple molecular substances (e.g., H₂O, CO₂) have low melting and boiling points. This is because only weak forces between the molecules (intermolecular forces) need to be overcome to change state, not the strong covalent bonds within them.
- Giant covalent structures (e.g., diamond, silicon dioxide) have very high melting and boiling points because a large amount of energy is needed to break the numerous strong covalent bonds throughout the lattice.
- Most covalent substances do not conduct electricity in any state because they lack delocalised electrons or free-moving ions to carry charge.
- Graphite is a critical exception. It is a giant covalent structure that is soft (layers can slide) and conducts electricity (due to delocalised electrons between layers).
Definitions
- Covalent Bond
- A chemical bond formed by the sharing of one or more pairs of electrons between two atoms.
- Simple Molecular Structure
- A structure composed of discrete molecules where atoms within each molecule are joined by strong covalent bonds, but the forces between separate molecules are weak.
- Giant Covalent Structure
- A structure consisting of a vast number of atoms joined by a continuous network of strong covalent bonds, forming a single giant lattice.
Worked example
The properties of four substances are listed below. Which substance is a giant covalent structure? | Substance | Melting Point (°C) | Electrical Conductivity (as solid) | Electrical Conductivity (as liquid) | |---|---|---|---| | P | -114 | Poor | Poor | | Q | 801 | Poor | Good | | R | 3652 | Good | Good | | S | 1710 | Poor | Poor |
- 1
First, establish the expected properties of a giant covalent structure.
They have very high melting points because strong covalent bonds must be broken.
They are typically poor electrical conductors as they have no free-moving charged particles.
- 2
Analyse substance P:
Its very low melting point (-114 °C) is characteristic of a simple molecular substance.
- 3
Analyse substance Q:
Its high melting point (801 °C) and ability to conduct only when liquid are classic properties of an ionic compound.
- 4
Analyse substance R:
Its very high melting point and good conductivity in both states are characteristic of a metal or graphite.
Since it conducts as a solid, it could be graphite.
- 5
Analyse substance S:
It has a very high melting point (1710 °C) and is a poor conductor in both states.
This combination of properties perfectly matches a typical giant covalent structure like silicon dioxide or diamond.
- 6
Therefore, S is the giant covalent structure.
Answer: S
Common mistakes
- ×Confusing the reason for low boiling points in simple molecules. It is the weak intermolecular forces *between* molecules that break, not the strong covalent bonds *within* them. This is a very common exam trap.
- ×Mixing up the properties of the two covalent structure types. Remember: simple molecules = low melting point; giant structures = high melting point.
- ×Forgetting the unique properties of graphite. Although it has a giant covalent structure and a high melting point, it is soft and conducts electricity, unlike diamond or silicon dioxide.
No-calculator tips
- ✓Use melting points as a primary sorting tool. If the value is negative or below a few hundred °C, think 'simple molecular'. If it's in the thousands, think 'giant structure' (covalent, ionic, or metallic).
- ✓Electrical conductivity is a powerful clue to structure type. No conductivity at all points towards simple molecular or most giant covalent structures. Conductivity only when molten points to ionic. Conductivity when solid points to metallic or graphite.